linux-kernel - Re: [PATCH v4: kvm 2/4] Kill the confusing tsc_ref_khz and ref

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Date:	Fri, 09 Oct 2009 22:47:52 +0200
From:	Jan Kiszka <jan.kiszka@....de>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v4: kvm 2/4] Kill the confusing tsc_ref_khz and ref_freq
 variables.

Zachary Amsden wrote:
> On 10/09/2009 10:36 AM, Jan Kiszka wrote:
>> Zachary Amsden wrote:
>>   
>>> On 10/08/2009 01:18 PM, Jan Kiszka wrote:
>>>     
>>>> Zachary Amsden wrote:
>>>>
>>>>       
>>>>> They are globals, not clearly protected by any ordering or locking,
>>>>> and
>>>>> vulnerable to various startup races.
>>>>>
>>>>> Instead, for variable TSC machines, register the cpufreq notifier and
>>>>> get
>>>>> the TSC frequency directly from the cpufreq machinery.  Not only is it
>>>>> always right, it is also perfectly accurate, as no error prone
>>>>> measurement
>>>>> is required.
>>>>>
>>>>> On such machines, when a new CPU online is brought online, it isn't
>>>>> clear what
>>>>> frequency it will start with, and it may not correspond to the
>>>>> reference, thus
>>>>> in hardware_enable we clear the cpu_tsc_khz variable to zero and make
>>>>> sure
>>>>> it is set before running on a VCPU.
>>>>>
>>>>> Signed-off-by: Zachary Amsden<zamsden@...hat.com>
>>>>> ---
>>>>>    arch/x86/kvm/x86.c |   26 ++++++++++++++++----------
>>>>>    1 files changed, 16 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>>> index 15d2ace..de4ce8f 100644
>>>>> --- a/arch/x86/kvm/x86.c
>>>>> +++ b/arch/x86/kvm/x86.c
>>>>> @@ -1326,6 +1326,8 @@ out:
>>>>>    void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>>>>    {
>>>>>        kvm_x86_ops->vcpu_load(vcpu, cpu);
>>>>> +    if (unlikely(per_cpu(cpu_tsc_khz, cpu) == 0))
>>>>> +        per_cpu(cpu_tsc_khz, cpu) = cpufreq_quick_get(cpu);
>>>>>        kvm_request_guest_time_update(vcpu);
>>>>>    }
>>>>>
>>>>> @@ -3061,9 +3063,6 @@ static void bounce_off(void *info)
>>>>>        /* nothing */
>>>>>    }
>>>>>
>>>>> -static unsigned int  ref_freq;
>>>>> -static unsigned long tsc_khz_ref;
>>>>> -
>>>>>    static int kvmclock_cpufreq_notifier(struct notifier_block *nb,
>>>>> unsigned long val,
>>>>>                         void *data)
>>>>>    {
>>>>> @@ -3072,14 +3071,11 @@ static int kvmclock_cpufreq_notifier(struct
>>>>> notifier_block *nb, unsigned long va
>>>>>        struct kvm_vcpu *vcpu;
>>>>>        int i, send_ipi = 0;
>>>>>
>>>>> -    if (!ref_freq)
>>>>> -        ref_freq = freq->old;
>>>>> -
>>>>>        if (val == CPUFREQ_PRECHANGE&&   freq->old>   freq->new)
>>>>>            return 0;
>>>>>        if (val == CPUFREQ_POSTCHANGE&&   freq->old<   freq->new)
>>>>>            return 0;
>>>>> -    per_cpu(cpu_tsc_khz, freq->cpu) = cpufreq_scale(tsc_khz_ref,
>>>>> ref_freq, freq->new);
>>>>> +    per_cpu(cpu_tsc_khz, freq->cpu) = freq->new;
>>>>>
>>>>>        spin_lock(&kvm_lock);
>>>>>        list_for_each_entry(kvm,&vm_list, vm_list) {
>>>>> @@ -3120,12 +3116,14 @@ static void kvm_timer_init(void)
>>>>>    {
>>>>>        int cpu;
>>>>>
>>>>> -    for_each_possible_cpu(cpu)
>>>>> -        per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
>>>>>        if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
>>>>> -        tsc_khz_ref = tsc_khz;
>>>>>            cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block,
>>>>>                          CPUFREQ_TRANSITION_NOTIFIER);
>>>>> +        for_each_online_cpu(cpu)
>>>>> +            per_cpu(cpu_tsc_khz, cpu) = cpufreq_get(cpu);
>>>>>
>>>>>          
>>>> This doesn't build for !CONFIG_CPU_FREQ.
>>>>
>>>>        
>>> And did it before?
>>>      
>> Yes, because cpufreq_get, which is undefined without CONFIG_CPU_FREQ,
>> did not exist so far. One may argue that this is a deficit of the
>> cpufreq API. However, it needs fixing.
>>    
> 
> I'll send a patch today.  I'm going with API deficient and will fix
> that, but this means I will also require a fix for kvm-kmod :(

That won't be tricky, I will queue up a fix.

BTW, is the KVM prepared for cpufreq_get returning 0?

Jan


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